The present invention relates to a method for producing ZnS particles having a coating formed of metal oxide, selected from SiO2, TiO2, Al2O3 or mixtures thereof with a content of cobalt2+, the Co/metal oxide/ZnS composite particles obtained hereby, and use thereof.
Zinc sulphide is a frequently used pigment. It has a low Mohs hardness of approximately 3 and is used in white pigments, for example lithopones, due to its high refractive index (n˜2.37); in cadmium pigments, zinc sulphide is used to vary the colour tones. Zinc sulphide doped with copper, silver, manganese or aluminium is used as a phosphor.
In order to avoid the greying of zinc sulphide caused by the influence of light, a cobalt salt is added to zinc sulphide. This greying of the ZnS in the presence of light and water is a photolysis reaction, which takes place at the surface of the zinc sulphide. The separation of elemental zinc leads to this grey colouring of the Co-free zinc sulphide. A doping with cobalt, by contrast, leads to light-fast zinc sulphides. ZnS products from the prior art contain up to 300-350 ppm Co2+. In accordance with a long-term effort ordered by the REACH Commission, cobalt-containing compounds must be increasingly substituted by non-toxic compounds. On the basis of the REACH regulations, it will also be necessary to identify separately those cobalt-containing compounds having >250 ppm CoS (=160 ppm Co2+).
A series of zinc sulphides treated with SiO2 are known in the prior art. Examples of this prior art include CN101177551, JP2005171316, JP11335823 and EP1380670. The materials disclosed therein however are mixed composites of ZnS and SiO2 or ZnS coated with SiO2.
As is known in the prior art, there is a link between the cobalt concentration in the particle and the light-fastness of the materials. Cobalt contents less than 190 ppm Co2+ lead to light-sensitive products.
Accordingly, AT 235438B describes a zinc sulphide pigment that is suspended in an aqueous solution of an alkali salt of the isopoly acids of silicon or phosphorous, and this suspension is flocked with a solution of a mixture of alkaline earth chloride and/or aluminium chloride, and the encased pigment is then filtered off, washed out, dried and ground, wherein a flocking solution is used that additionally contains at least one salt of the metals iron, cobalt, nickel, copper, cadmium, titanium, zirconium and cerium, and of the rare earth metals, especially chlorides or nitrates, in a percentage of at least 0.03% by weight, especially 0.05-0.5% by weight metal, based on the pigment.
No ZnS products having such a reduced cobalt content of less than 190 ppm Co2+ with simultaneous resistance to greying are known in the prior art. There is thus a need for greying-resistant zinc sulphide of this type.
The inventors have considered the problem of reducing the cobalt content in zinc sulphide whilst at the same time not affecting the light-fastness disadvantageously. The inventors have found that zinc sulphide must have optimised properties in particular at the surface of the particles, where photolysis occurs, in order to be able to reduce the cobalt content. The inventors have been able to prove that the majority of the cobalt is bonded only at the surface of the ZnS. The inventors have found that cobalt atoms in the interior of a ZnS particle are not responsible for the photostabilisation of the ZnS and that only cobalt atoms at the surface of the particles can contribute to the prevention of greying. Indeed, the mechanism of photolysis and the role of the cobalt have been debated in the prior art, however it is currently assumed that the oxidation levels of cobalt change partially during the photolysis process.